JPH0337351B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise reduction device for reducing noise in an image signal, and particularly to an improved noise reduction performed using a low-pass filter.

Generally, in a type of noise reduction device that uses a frame memory to reduce noise in an image signal based on inter-frame correlation of the image signal,
The noise reduction rate for moving image portions decreases. That is, it is well known that if the noise reduction rate is not lowered in a moving image portion, secondary deterioration of the image signal will occur due to noise reduction. One of the solutions to this problem is as described in Japanese Patent Application Laid-Open No. 1983-37018, which is filed by the applicant.
Although a motion correction method is introduced, even if motion correction is applied to a moving image portion, there may still be cases where noise cannot be sufficiently reduced in the moving image portion.

In the above case, instead of reducing noise by using inter-frame correlation of image signals, noise must be reduced by signal processing within the same frame, and such intra-frame noise reduction is difficult. In a nutshell, the principle is to reduce noise using a spatial low-pass filter, but simply using a low-pass filter will result in blurring of the reproduced image caused by the image signal. arise. To make matters worse, for television images, removing noise components above 1-2 _MHz does not significantly improve the subjective signal-to-noise ratio, so low-pass filters used for noise reduction must have a fairly narrow band. Therefore, if such a low-pass filter is simply used, the blur of the reproduced image will become severe.

In response, several devices have been proposed that reduce noise by processing image signals within frames without increasing the blurring of reproduced images. There are well-known Wiener filters that remove noise by focusing on differences in the statistical properties of However, it has been difficult to apply this method to processing television image signals.

Further, as a conventional noise reduction device based on intra-frame processing of an image signal, one that utilizes visual characteristics has also been proposed. In other words, if the edge portions of the reproduced image are blurred, it will be perceived as visually significant, so if a low-pass filter is used that has a relatively wide passband for the edge portions and a narrow passband for the flat portions of the image, This results in more noise components remaining in the edge area than in the flat area, but the visual characteristics of the edge area are degraded compared to the flat area. The feeling of blur can be reduced.
Therefore, although a low-pass filter adapted to such visual characteristics is highly suitable for noise reduction in image signals, the implementation of such a filter requires nonlinear circuit elements according to conventional filter configurations, and furthermore, The drawback was that it required a large-scale circuit configuration.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned conventional drawbacks and to provide a noise reduction device that implements a low-pass filter having characteristics that nearly match the visual characteristics of images with a relatively simple circuit configuration. .

That is, the noise reduction device of the present invention is a noise reduction device that reduces noise components of an input image signal using a one-dimensional or two-dimensional low-pass filter. edge component detection means for detecting horizontal and vertical edge components of the input image signal consisting of the absolute value of the difference between the input and output image signals of the delay element having a delay time equal to the color subcarrier period; a subtracter that forms a difference between the input and output image signals; a multiplier that multiplies the difference between the input and output image signals in a stepwise manner by multiplying the difference in proportion to the edge component; an adder for adding the filtered output image signal of the low-pass filter to the filtered output image signal of the low-pass filter, the component removed by the low-pass filter being substantially proportional to the detection result of the edge signal component of the input image signal. The filtered output image signal is added stepwise to reduce the sharpness reduction of the filtered output image signal.

The invention will be explained in detail below by way of example embodiments with reference to the drawings.

An example of the configuration of the noise reduction device of the present invention is shown in FIG. In the illustrated configuration, reference numeral 1 denotes a two-dimensional low-pass filter having appropriate band characteristics capable of sufficiently reducing noise components for flat parts of the reproduced image; It is called. Further, reference numeral 2 denotes a horizontal edge detecting section capable of obtaining a detection output signal proportional to the horizontal edge component of the image signal, and is constructed, for example, as will be described later with reference to FIG. Further, numeral 3 denotes a vertical edge detecting section capable of obtaining a detection output signal proportional to the vertical edge component of the image signal, and is constructed, for example, as will be described later with reference to FIG. The horizontal and vertical edge detection output signals are mixed in an adder 4 to form an edge component signal of the image signal.

Therefore, in the configuration example shown in the first diagram, the image signal from the input terminal is supplied in parallel to the low-pass filter 1, the horizontal edge detection section 2, and the vertical edge detection section 3, and the wave output of the low-pass filter 1 is The image signal and the input image signal are supplied to a subtracter 5, and the wave output image signal, which has less noise but is blurred, is subtracted from the input image signal which is not blurred but has more noise, and is removed by the low-pass filter 1. The components, that is, the noise component in the flat part and the edge signal component containing noise, are extracted and supplied to the multiplier 6, and multiplied by the edge component signal from the adder 4 to obtain the edge signal component containing noise. The edge signal component containing noise is extracted in gated form as the multiplication output signal, and the gate output edge signal component is supplied to the adder 7 to generate the wave output image signal from the low-pass filter 1. A wave output image signal in which the noise is also recovered, but the edge signal component is recovered, that is, an image signal from which at least the noise in the flat part has been sufficiently removed, that is, the low-pass filter wave output image in the desired form as described above. Taking out the signal.

That is, if the ratio β of the edge signal component to the input image signal A _i is 0≦β≦1, the low-pass filter wave output image signal A _p corrected by the configuration shown in Figure 1 is the normal low-pass filter wave output image. signal
It can be expressed as the following equation (1) for A _l .

A _p = A _l + β (A _i − A _l ) = β A _i + (1 − β) A _l (1) Therefore, in the noise reduction device of the present invention, when edge portions where noise is not noticeable are removed, the image It is possible to obtain a low-pass filter wave output image signal from which noise is sufficiently removed from flat areas where noise is noticeable, without suppressing edge signal components that cause significant blurring.

Note that in the above equation (1), the input image signal A _i
and the low-pass filter output image signal A _l have a linear relationship, but as shown in FIG. , the signal level of the edge component signal to be multiplied is non-linearly modified in accordance with the visual characteristics, and the noise recovery at the edges is suppressed by multiplying the edge component signal in a form that suppresses low-level noise components. When the device of the present invention is configured in a digital format,
The nonlinear level converter 8 described above is stored in a fixed storage device,
That is, it can be easily configured using a so-called ROM.

Next, another configuration example of the noise reduction device of the present invention in the case of reducing noise in a color image signal will be described.

First, an example of the configuration of the horizontal edge detection section 2 described above is shown in FIG. For example,
In the NTSC color image signal, after mutually subtracting and removing color subcarrier components that have the same phase every two lines, a horizontal edge component signal consisting of a frequency component of the same level as the color subcarrier component is detected. That's what I do. However, since the horizontal edge component signal detected with this configuration has a time delay of a little more than one line with respect to the input image signal, it is necessary to add an appropriate delay line to each circuit in the configuration shown in the first figure. need to be addressed.

FIG. 4 shows an example of the configuration of the vertical edge detection section 3 described above. When noise reduction is performed by digitizing a color image signal at a sampling frequency of 3 x _sc with respect to a color subcarrier frequency _sc , the input A digital color image signal is supplied to a three-stage D-flip-flop 12, 12', 12'', and the input/output image signal is supplied to a subtracter 13, and the color subcarrier component is removed as the subtracted output signal. The edge component signal is taken out via the absolute value circuit 13. Note that the vertical edge component signal detected with this configuration also has a time delay with respect to the input color image signal, so as described above,
It is necessary to add appropriate delay elements to each circuit of the noise reduction device of the present invention.

FIG. 5 shows a configuration example of the apparatus of the present invention in which the color subcarrier component is removed and noise reduction is performed in the same manner as described above. In the illustrated configuration example, as is clear from the above, delay elements 15, 16, and 17 are added to the circuit configuration similar to that shown in the first figure to correct the time delay occurring in the edge component detection output signal described above. It is planned to be applied.

Although the configuration shown in FIG. 5 is used to reduce the noise of the composite color image signal, it goes without saying that the configuration shown in FIG. 1 can also reduce the noise of monochrome image signals and component color image signals.

As is clear from the above description, according to the present invention, it is possible to easily configure a low-pass filter with desired characteristics that also has non-linear characteristics with a relatively simple circuit configuration, and by using the low-pass filter. By only performing signal processing within the field, sufficient noise reduction can be performed even on moving images without using interframe correlation.

Although the device of the present invention was directly intended to be used to assist in interframe signal processing performed as a motion compensation noise reduction device, considerable effects can be expected even when the device of the present invention is used alone. .

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a configuration example of the noise reduction device of the present invention, FIG. 2 is a block diagram showing another configuration example of a part thereof, and FIG. 3 is a block diagram showing an example of the configuration of the horizontal edge portion. FIG. 4 is a block diagram showing an example of the structure of the vertical edge portion, and FIG. 5 is a block diagram showing another example of the structure of the noise reduction device of the present invention. 1...Low frequency filter, 2...Horizontal edge section, 3
... Vertical edge section, 4, 7 ... Adder, 5, 1
1, 13... Subtractor, 6... Multiplier, 8... Level converter, 9... 2 line delay line, 10, 14...
...Absolute value circuit, 12, 12', 12''...D flip-flop, 15, 16, 17...Delay element.

Claims (1)

[Claims] 1. In a noise reduction device that reduces noise components of an input image signal using a one-dimensional or two-dimensional low-pass filter, the absolute value of the difference between the input and output image signals of the two-line delay line and the color edge component detection means for detecting horizontal and vertical edge components of the input image signal, which are formed by the absolute value of the difference between the input and output image signals of a delay element having a delay time equal to the carrier wave period; and input and output of the low-pass filter. a subtracter that forms a difference between image signals; a multiplier that multiplies the difference between the input and output image signals in a stepwise manner by multiplying the difference in proportion to the edge component; and a multiplier that outputs the multiplied output image signal of the multiplier as the filter output an adder for adding to the image signal, the component removed by the low-pass filter is added to the filtered output image signal of the low-pass filter in approximately proportion to the detection result of the edge signal component of the input image signal. A noise reduction device characterized in that a reduction in sharpness of the filtered output image signal is reduced by adding the filtered image signal in stages. 2. The noise reduction device according to claim 1, in which mixing of the color subcarrier components is prevented based on the correlation of color subcarrier components for each of a plurality of lines and for each of a plurality of fields in the input image signal, and horizontal and vertical A noise reduction device characterized in that the edge signal component of the noise reduction device is detected.